Trejos served as an independent consultant and parliamentary adviser. He joined the Foreign Ministry as an advisor to the Vice Minister in the 1998–2000 period, where he served as Foreign Policy Director, where he also served as liaison with the Legislative Assembly of Costa Rica and in the trade talks with the European Union. Between 2000 and 2002, he moved to the Ministry of Tourism, where he was chief of staff at the minister's office.[4]
In 2003 he was in charge of negotiation between Costa Rica and Nicaragua for the Procuenca del Río San Juan Project and as a researcher of the Program of High Public Management of INCAE in 2004. He joined the Citizens' Action Party in 2005 and served as an advisor in Deputy Ruth Montoya Rojas' office. Later, he was Director of Advisors for the Citizen Action Party's bench and a principal advisor to the group's spokesperson. Between 2010 and 2014, he was an independent consultant for the Information and Knowledge Society Program (PROSIC) of the Universidad de Costa Rica, the Inter-American Development Bank and the Institute of Development Studies.[4]
He assumes the position of Costa Rican ambassador in Managua, Nicaragua, as of June 8, 2016 after the previous holder, Javier Sancho Bonilla, retired. In addition to streamlining visa delivery processes at the Nicaraguan consulate, through the implementation of a prior appointment process, Trejos promoted trade and tourism exchange among the two nations.[7] Trejos was appointed Director of the Intelligence and Security Directorate of Costa Rica in the administration of Carlos Alvarado Quesada. This is the intelligence agency of Costa Rica.